1. Field of the Invention
This invention is concerned with an improvement in or relating to a magnetic brush cleaning device for use in an electrostatic reproducing apparatus or electrostatic recording apparatus using a powder toner. More specifically, the present invention relates to an improvement in a cleaning device of the type in which a magnetic attraction roller having a surface for attracting a magnetic powder is disposed so as to face the surface of a toner image retaining member, the height of a brush-like ear of the magnetic powder attracted to the magnetic attraction roller is restricted by ear restriction means and the brush-like ear of the magnetic powder frictionally slides on the surface of the toner image retaining member, thereby removing the toner on the toner image retaining member therefrom.
2. Description of the Prior Art
The magnetic brush cleaning device of the kind described above is disclosed in U.S. Pat. No. Re 28,566 (reissued Oct. 7, 1975), for example. In the magnetic brush cleaning device of this kind, a bias voltage of a polarity to attract electrically the toner on the toner image retaining surface is applied to the surface of a magnetic attraction roller so that the brush-like ear of the magnetic powder formed on the surface of the magnetic attraction roller, that is, the magnetic brush, collects the toner. The cleaning device does not call for an exhaust device or a filter required in the case of a fur brush cleaning device so that not only the cleaning device itself but also the reproducing apparatus or recording apparatus can be simplified in construction and be reduced in size and weight. Another advantage provided by the cleaning device is that contamination of the reproducing apparatus or recording apparatus by the scattering toner is considerably less and damage to the surface of the toner image retaining member which is often observed in a blade cleaning device is extremely unlikely. However, a cleaning device of this kind has a drawback in that the toner on the toner image retaining member can not be removed reliably from time to time or, the cleaning operation becomes sometimes unreliable.
In order to ensure the reliable cleaning operation, the magnetic brush must reliably isolate and collect the toner attaching to the toner image retaining member therefrom. To this end, the height of the magnetic brush and the relation of distance between the surface of the magnetic attraction roller and the surface of the toner image retaining member are of the utmost importance. The conventional magnetic brush cleaning devices mostly use an electrically conductive carrier having resistivity of 10.sup.3 Ohms to 10.sup.10 Ohms as the magnetic carrier. In this case, the following problems occur.
(1) Charge injection is likely to occur to the toner on the surface of the toner image retaining member from the surface of the magnetic attraction roller via the magnetic brush, and the toner to which the charge is thus injected can not be collected easily by the magnetic brush, so that setting of the bias voltage to be applied to the surface of the magnetic attraction roller becomes difficult. PA0 (2) If fine pinholes or scratches exist on the surface of the toner image retaining member, the surface of the magnetic attraction member is short circuited to the toner image retaining member via the magnetic brush; hence, the effective bias voltage can not be applied to the surface of the magnetic attraction roller and the magnetic brush can not as a whole collect the toner. PA0 (3) When the magnetic brush collects the toner and the mixing ratio of the magnetic carrier and the toner changes in the magnetic brush, the resistance of the magnetic brush changes so that the bias voltage on the surface of the magnetic attraction roller changes and toner collection by the magnetic brush can not be effected stably. PA0 (1) A resin film or oxide film having high resistivity is disposed on the surface of the magnetic attraction roller so as to prevent discharge through the magnetic brush; and/or PA0 (2) A method which controls the bias voltage so that even when discharge through the magnetic brush occurs, the surface of the magnetic attraction roller holds an effective voltage.
To cope with these problems, the following methods have been employed:
However, the first method (1) creates a new problem in that the toner adheres to the resin film or oxide film on the surface of the magnetic attraction roller and the charge must be removed. The second method (2) makes the apparatus more complicated in construction and increases the cost of production.
The problems encountered in using the conductive carrier do not occur when an insulating carrier obtained by coating the magnetic carrier with an insulating resin is used as the carrier. In the case of the conductive carrier, however, the carrier itself that forms the magnetic brush plays the role of the opposed electrode that electrically attracts the toner on the toner image retaining member. The insulating carrier can not play such a role so that the magnetic attraction roller that forms the opposed electrode must be disposed closer to the surface of the toner image retaining member than the conductive carrier. Moreover, the bias voltage to be applied upon the surface of the magnetic attraction roller must be set to a higher level than in the case of the conductive carrier.